This invention relates generally to a method of detecting effluents from a gas chromatographic column by photoionization and in particular to a photoionization detector of which the photon source is a microwave induced helium plasma.
The photoionization detector is generally constructed in such a manner that photons are generated by a discharge in the presence of a discharge gas and guided into a detecting section to ionize the sample gas, the sample gas ions being collected by an electrode in the detection section for the measurement of ionization current. While compounds such as O.sub.2, N.sub.2, H.sub.2, H.sub.2 O, SO.sub.2 and NO.sub.2 give little or no response in a flame ionization detector, a photoionization detector is capable of detecting both inorganic and organic constituents with a high sensitivity and hence is suited for gas chromatography.
Prior art photoionization detectors can be divided generally into the following two classes, the first utilizing a low pressure discharge lamp as the photon source and the second using atmospheric pressure discharges. The disadvantage of a sealed source in the case of the former is that the highest photon energy that is available for ionization is limited to about 12eV by the transmission of the window. Since the ionization potential of O.sub.2 is 12.08 eV, this means that the detector sensitivity cannot be good for this compound. N.sub.2 and H.sub.2 with ionization potentials respectively of 15.58 eV and 15.61 eV will not even give a response.
To overcome these limitations, atmospheric pressure discharges in helium were used as light sources including the 21.22 eV radiation from He I line. Since the discharge occurs at atmospheric pressure, no windows are necessary and this allows higher energy photons to be utilized in the ionization process. A direct current discharge detector of this type with a pointed cathode and a conical anode is disclosed, for example, in U.S. Pat. No. 3,418,514 issued to J. C. Sternberg. This approach, however, has the disadvantage that the anode is quickly destroyed by the sputtering action of the current of electrons that impinge onto its surface. This results in an increase in noise due to the arc moving around on the surface of the anode.
An attempt to overcome this problem is found in U.S. Pat. No. 4,266,196 issued to K. Kawazoe et al which describes a direct current discharge photoionization detector with a point cathode and a disk anode, the claim being that the electron current to the anode in this system is distributed around the edge of the disk and therefore that erosion and noise due to sputtering are reduced. In practice, however, this method fails because sputtering will eventually cause one particular area of the disk to have locations on the surface where the electric field strength is concentrated and hence higher than in the surrounding areas. The distributed arc will then collapse and the sputtering rate at such locations will be accelerated, causing the condensed arc to wander about the anode surface and resulting in an increased noise level.